1. Field of the Invention
The present invention relates to control method and device thereof, and more particularly, to a control method capable of preventing flicker effect and a related light emitting device.
2. Description of the Prior Art
Light emitting diodes (LEDs) offer advantages of energy savings, long device lifetime, no mercury used, high achievable color gamut, without idle time, and fast response speed, so that LED technology is widely applied in fields of display and illumination. In addition, compared with a conventional light source device, light emitting diodes are suitable for fabrication as a tiny device or an array device, such as in traffic lights, outdoor displays, backlight modules of liquid crystal displays, PDAs, notebooks, or mobile phones with features of small size, shock resistance, ease of mass production, and high applicability.
Please refer to FIG. 1, which is a schematic diagram of an LED driving device 10 according to the prior art. The LED driving device 10 is utilized for driving a light source module 102 which includes a plurality of LED groups C1 to Cm arranged in parallel. The LED driving device 10 includes a voltage converter 104, a current source 106, a pulse modulation unit 108, and a control unit 110. The voltage converter 104 is utilized for providing an output voltage VD to the light source module 102. The current source 106 is utilized for providing driving currents ID1 to IDM for LED groups C1 to Cm to drive the light source module 102. The pulse modulation unit 108 is utilized for dimming according to a dimming signal SD. In general, a plurality of headroom voltages VHR1 to VHRm exist on each path of the LED groups C1 to Cm. The headroom voltages VHR1 to VHRm represent the voltage value across the current source 106 on each path of the LED groups C1 to Cm, i.e. available voltage value for the current source 106 on each LED group path. In practice, the currents passing through the LEDs can usually be kept constant, i.e. the driving currents ID1 to IDM are fixed, for steady brightness control and power consumption of the LEDs. However, the voltages across the LEDs may not be all the same due to non-ideal factors in the manufacturing process or other reasons, and the headroom voltages VHR1 to VHRm are not the same correspondingly. In such a condition, the headroom voltage may be too high or too low, and will result in some unwanted effects. For example, if the headroom voltage is too high, the power consumption of the current source will increase, and the power conversion efficiency will be reduced. If the headroom voltage is not high enough, the current source will operate in an improper state, and cannot keep constant current sink, even to the point of not being able to provide the required driving current to the LED, and the LED will not conduct.
Therefore, as shown in FIG. 1, in the conventional technology, the voltage converter 104 may be controlled to change the output voltage VD by the control unit 110 in negative feedback form in order to obtain appropriate headroom voltages. The control unit 110 includes a voltage selector 112, an error amplifier 114, and a conversion controller 116. The voltage selector 112 is coupled to the output terminal of each LED group C1 to Cm for selecting one of the headroom voltages VHR1 to VHRm as the feedback voltage VFB. Again, the feedback voltage VFB and a predetermined reference voltage VREF are inputted to the positive end and negative end of respectively. The error amplifier 114 generates an error voltage signal SE according to the difference between the feedback voltage VFB and the predetermined reference voltage VREF. Furthermore, the conversion controller 116 generates a voltage control signal SC according to the error voltage signal SE for control the conversion process of the voltage converter 104. Thus, as the headroom voltages VHR1 to VHRm corresponding to each LED group C1 to Cm are too low, the error amplifier 114 generates the error voltage signal SE sent to the conversion controller 116, and the conversion controller 116 generates the voltage control signal SC accordingly to control the voltage converter 104 to increase the output voltage VD. As a result, as the driving currents ID1 to IDM are fixed, the headroom voltages VHR1 to VHRm will not vary accordingly. On the other hand, the headroom voltages VHR1 to VHRm are proportional to the output voltage VD. Therefore, the control unit 110 is able to control the output voltage VD to be increased so that the headroom voltages VHR1 to VHRm increase correspondingly, and vice versa. Therefore, under the steady driving currents ID1 to IDM provided, the LED driving circuit 10 can lock the headroom voltages VHR1 to VHRm within an appropriate range, such as the predetermined reference voltage VREF, by the control unit 110.
However, current variation situations may occur often in the currents passing through the LEDs in many cases. For example, during the dimming process, the brightness of the LEDs can be changed by adjusting the currents passing through the LEDs (i.e. by adjusting the driving currents ID1 to IDM), so that the voltages across the LEDs vary correspondingly. But, the LED driving circuit 10 adjusts the output voltage VD by only comparing the output voltage VD with a fixed predetermined reference voltage, which results in consuming too much feedback tracking time for adjusting the output voltage VD. In other words, the output voltage VD can not be arranged to an appropriate voltage level immediately, and the headroom voltages of the current source 106 become too low to provide sufficient driving currents, so that flicker effects occur.